Automobiles require shiftable transmissions as a result of the physics of the internal combustion engine. All internal combustion engines have a maximum engine rotational speed above which the internal combustion engine can not effectively operate due to limitations of various internal combustion engine components, such as the valvetrain or reciprocating components. Additionally, internal combustion engines have an engine speed range where horsepower and torque are at their maximum. The transmission, through changes in gear ratio, allows the ratio between the rotational speed of the engine and the drive wheels to change as the automobile accelerates or decelerates. By varying the gear ratios within the transmission, the internal combustion engine may operate below the maximum rotational speed and preferably near the engine speed range for best performance and/or fuel economy. The transmission provides a gear set operable to provide a reverse gear for backing the automobile. The transmission also provides a low gear, effective to allow adequate acceleration of the automobile while not sacrificing the top speed of the automobile.
Vehicular transmissions typically employ a clutch mechanism such as a hydraulically actuated plate-type clutch or a set of synchronizers in order to effect a gear selection. Engineers strive to make this gear selection as smooth and imperceptible to the driver as possible. Modern vehicular transmissions are designed with a very small footprint to allow tighter packaging within the engine bay resulting in an increased passenger cabin volume. Therefore, it is important to ensure that the usable space within the transmission housing is effectively utilized in the design of the transmission.
Automatically and manually shiftable transmissions constitute the two main varieties of vehicular transmissions. Automatically shiftable transmissions select the gear ratio with no input from the driver, whereas the manually shiftable transmissions require a gear selection input from the driver. Different gear selection techniques exist for manual transmissions such as, dual clutch shifting and synchronized shifting. The synchronized transmission designs utilize synchronizers to ensure that the drive gear rotational speed closely matches that of the input shaft during a shift to effect a smooth transition through the gear sets. The dual clutch transmission, or DCT, employs two separate input shafts, each with its own corresponding clutch, to engage and disengage the input shafts. In a 5-speed DCT, for instance, one input shaft would account for forward ranges 1, 3, and 5, and the second shaft would account for forward ranges 2 and 4 as well as a reverse range. Automatically shiftable transmissions typically employ hydraulically actuated plate-type clutches. Other variations of the automatic transmission may employ both hydraulically actuated plate-type clutches as well as one-way clutches. In this variation, the one-way clutches are used to improve shift feel and shift timing by enabling the smooth release of the off-going clutching element as the on-coming element gains torque capacity.